Twenty-five patients are planned to be enrolled. After induction of anesthesia, insertion of the PAC and the esophageal-doppler probe, the patient are placed in the following successive positions: a) supine, b) head-down tilt, c) head-up tilt, d) supine, e) supine with phenylephrine administration f) pace heart rate 80 bpm, g) pace heart rate 110 bpm, and CO are measured simultaneously using the CardioQ and PAC TD.